1. Field of the Invention
The present invention relates to a driving circuit of a light-emitting element for an image display apparatus, more specifically to a driving circuit for an active matrix type light-emitting element for controlling a self-luminous element such as organic and inorganic electroluminescence elements and light-emitting diodes (hereinafter the “electroluminescence” being referred to as “EL”; the “light-emitting diode” being referred to as “LED”). The present invention relates also to an active matrix type display panel employing the aforementioned driving circuit.
2. Related Background Art
The display units which display characters with a dot matrix of light-emitting elements such as organic or inorganic EL light-emitting elements and LED combined in an array are widely used in televisions and portable terminals.
In particular, the displays employing a self-luminescent element are attracting attention, since such a display does not require a backlight for illumination and has a wide view angle and other features, differently from the display units employing liquid crystals. Among them, active matrix types of displays which are driven statically by combination of transistors or the like with the above light-emitting elements come to be noticed because of high luminance, high contrast, high fineness, and other superiority in comparison with simple matrix-driven display units conducting time-divisional driving.
The systems employing an organic EL element also include analog gradation systems, areal gradation systems, and time-controlled gradation systems similarly as conventional systems for gradation of the image.
(1) Analog Gradation System
As an example of conventional analog systems, FIG. 7 shows a simplest display element of an active matrix-driven light-emitting element which is provided with two thin film transistors (hereinafter being referred to as “TFT”) for one pixel. In FIG. 7, the numerals indicate the members as follows: 101, an organic EL element; 102 and 103, a TFT respectively; 108, a scanning line; 107, a signal line; 109, a power line; 110, a ground potential; and 111, a memory capacitance employing a condenser.
The operation of this driving circuit is explained below. With TFT 102 turned on by scanning line 108, an image data voltage from signal line 107 is accumulated in memory capacitance 111. Even after TFT 102 is turned off by turning-off of scanning line 108, the aforementioned voltage is kept applied to the control electrode of TFT 103 to keep TFT 103 in an ON state.
On the other hand, the first main electrode of TFT 103 is connected to power line 109; the second main electrode thereof is connected to the first electrode of the light-emitting element; and the control electrode thereof is connected to the second electrode of TFT 102 to input the image data voltage. The quantity of the electric current is controlled by the aforementioned image data voltage. Organic EL element 101, which is placed between power line 109 and grounding potential 110, emits light in accordance with the electric current quantity.
The above electric current quantity is controlled by the control voltage of TFT 103. The luminance of the light emission is changed by changing the current characteristics in an analog manner by utilizing the region where the characteristic of the first main electric current (Vg-Is characteristics) relative to the aforementioned control voltage rises (the region being referred to as the “saturation region”).
Consequently, the light emission luminance of the organic EL element as the light-emitting element is controlled to conduct display with gradation. This system of display with gradation is called an analog gradation system since the analog image data voltage is utilized.
The currently used TFTs include amorphous silicon (a-Si) type ones and polysilicon (p-Si) type ones. The polycrystalline silicon TFTs are becoming more important in view of the high mobility, possibility for fineness of the element, and possibility for low-temperature production process owing to the laser working technique progress. However, the polycrystalline silicon TFT is liable to be affected by the grain boundary of the constituting crystal grains, and tends to have the Vg-Is current characteristics varying among the TFT elements. As the results, even with uniform video signal voltage inputted to the elements, the display can be irregular disadvantageously.
Generally, most of TFTs are used merely as a switching element, and are used by application of control voltage much higher than the threshold voltage of the transistor in the region where the voltage of the second main electrode is constant relative to the voltage of the first main electrode (the region being called a linear region), whereby the variance is less liable to be caused in the aforementioned saturation region. On the other hand, this method utilizing the saturation region is liable to cause variance.
Further, in this system, the image data signal should be changed corresponding to the luminance-voltage characteristic of the organic EL element. Since the voltage-current characteristic of the organic EL element is similar to the nonlinear diode characteristic, the voltage-luminance characteristic has also a steep rise like the diode characteristic. Therefore, the image data signals should be treated for gamma correction, which makes the drive control system complicated.
(2) Areal Gradation System
An areal gradation system is proposed in a paper, AM-LCD2000, AM3-1. In this system, one pixel is divided into subpixels; the subpixels are turned on or off independently; and the gradation is expressed by the area of the pixels in an ON state. FIG. 8 shows a planar constitution of a pixel divided into six subpixels.
In such a system, the TFT can be driven at a control voltage much higher than the threshold voltage in the linear region where the voltage of the second main electrode is constant relative to the voltage of the first main electrode, whereby the TFT can be used with stable TFT characteristics, resulting in stable luminance of the light-emitting element. In this system, the respective elements are controlled to be on and off, and emit light at a constant luminance without gradation, the gradation being controlled by the area of the subpixels emitting light.
This system is limited to digital gradation because of the division into subpixels. To increase the levels of gradation, the number of division should be increased with decrease of the area of one subpixel. Even if the transistors are made finer by use of polycrystalline silicon TFT, the transistor portion of each of the pixel decreases the area of the light-emitting portion to lower the pixel aperture ratio, resulting in decrease of the luminance of the emitted light of the display panel. Therefore, increase of the numerical aperture lowers the gradation. Thus, the brightness and the gradation are in a relation of trade-off, so that the improvement of the gradation is not easy.
(3) Time-controlled Gradation System
The time-controlled gradation system controls the gradation by the light-emitting time of the organic EL element, as reported in a paper, 2000SID36.4L.
FIG. 9 shows an example of a circuit diagram of one pixel of a conventional display panel employing the time-controlled gradation system. In FIG. 9, the same reference numerals as in FIG. 7 are used for indicating the corresponding members. The numeral 104 indicates a TFT, and the numeral 112 indicates a reset line.
In the time-controlled gradation system employing this circuit construction, when TFT 103 is turned on, organic EL element 101 emits light at the highest luminance level by the voltage from power line 109. Then, TFT 104 is driven to turn TFT 103 on and off suitably and repeatedly with in a time of one field to display the gradation by the light-emitting time duration.
In this type of system, one field is divided into plural subfield periods, and the light-emitting time is controlled by the light-emitting periods. For example, to display 8 bits (256 gradation levels), the ratio of the light-emitting time is selected from eight subgroups of periods of 1:2:4:8:16:32:64:128. Immediately before each of the subfield periods, an addressing period is necessary for the scanning lines of the all of the pixels to select emission or no-emission of light in the respective subfields. After the addressing period, the voltages of source lines 109 are simultaneously changed to emit light from the entire face of the display panel.
Therefore, since the display is not conducted in the addressing period in principle, the effective light-emission period in one field for N-bit gradation display is represented by the relation below:(Effective light emission period)=(One field period)−(One picture addressing period×N)Thereby, the light emission time is made shorter, and the amount of the light emission is less for the observer.
To offset such disadvantages, it is desirable to increase the light-emission amount of one subfield to increase the light emission of the entire fields. For this purpose, the light-emission luminance of the respective light-emitting elements should be increased, which can result in shorter life of the light-emitting elements and other disadvantages. Further, although a usual liquid display (LCD) requires only one time of addressing for one field, this type of the gradation system requires the addressing times of the gradation bits for one field, which necessitates a higher speed of the addressing circuit.